1. Field of the Invention
The present invention relates to high-frequency filters, complex electronic components using the filters, portable radio apparatuses using the filters or the components, and a frequency adjustment method for a high frequency filter and more particularly, to a high-frequency filter formed of a plurality of transmission lines side-coupled in a plurality of stages, a complex electronic component using the filter, a portable radio apparatus using the filter or the electronic component, and a frequency adjustment method for a high frequency filter.
2. Description of the Related Art
In recent years, compact, high-performance portable radio apparatuses have been increasingly developed in the mobile communication field. To this end, a high-frequency circuit needs to have higher performance. Among high-frequency circuits, a high-frequency filter, which has a great effect on the performance of a portable radio apparatus, is strongly required to be made compact and to have higher performance. A conventional side-coupled high-frequency filter using a transmission line will be described below. FIG. 12 is a block diagram of a conventional side-coupled high-frequency filter. A high-frequency filter 50 serving as a bandpass filter has three transmission lines SL51 to SL53 which are side-coupled in three stages. The transmission lines SL51 to SL53 are connected in parallel to capacitors C51 to C53 for parallel resonance, respectively. One end of the transmission line SL51 is connected to an input terminal Pi through an input capacitor C54. One end of the transmission line SL53 is connected to an output terminal Po through an output capacitor C55. The other ends of the transmission lines SL51 and SL53 and one end of the transmission line SL52 are connected to the ground.
In the above conventional high-frequency filter, however, as shown in FIG. 13, the frequency characteristic is gentle at both sides of the center frequency, no pole exists at both sides of the center frequency, and attenuation is insufficient in the vicinity of the center frequency.
The present invention has been made to solve the foregoing problems. Accordingly, it is an object of the present invention to provide a high-frequency filter having a pole at either the higher-frequency or lower-frequency side of the center frequency and sufficient attenuation in its frequency characteristic, a complex electronic component using the filter, a portable radio apparatus using the filter or the electronic component, and a frequency adjustment method for a high frequency filter.
To solve the foregoing problems, the present invention provides a high-frequency filter including: a plurality of transmission lines side-coupled in a plurality of stages; capacitors for parallel resonance respectively connected in parallel to the plurality of transmission lines, an input terminal connected through an input capacitor to one end of an input transmission line among the plurality of transmission lines; an output terminal connected through an output capacitor to one end of an output transmission line among the plurality of transmission lines; and an inductor for forming a pole provided between the ground and the connection point where the other end of the input transmission line is connected to the other end of the output transmission line.
According to the above high-frequency filter, since the other end of the input transmission line is connected to the other end of the output transmission line and the inductor for forming a pole is provided between the connection point and the ground, a pole is formed at the higher-frequency or lower-frequency side of the center frequency and sufficient attenuation is ensured.
The present invention also provides a high-frequency filter including: a plurality of transmission lines side-coupled in a plurality of stages; an input terminal connected to one end of an input transmission line among the plurality of transmission lines; and an output terminal connected to one end of an output transmission line among the plurality of transmission lines, wherein the other end of the input transmission line is connected to the other end of the output transmission line to form a closed circuit including the plurality of transmission lines between the input transmission line and the output transmission line; and the connection point between the other end of the input transmission line and the other end of the output transmission line is connected to the ground through an inductor for forming a pole.
According to the above high-frequency filter, since the other end of the input transmission line is connected to the other end of the output transmission line to form the closed circuit between the input transmission line and the output transmission line, and the connection point between the other end of the input transmission line and the other end of the output transmission line is connected to the ground through the inductor for forming a pole, due to a current flowing through the closed circuit, a pole is formed at the higher-frequency or lower-frequency side of the center frequency and sufficient attenuation is ensured.
The magnitude of the current flowing through the closed circuit can be changed by changing the inductance of the inductor for forming a pole. Therefore, since the frequency at the pole can be adjusted by the magnitude of the current flowing through the closed circuit, the attenuation at the pole at the higher-frequency or lower-frequency side of the center frequency and the position where the pole is formed can be easily changed.
The above high-frequency filter may be configured such that it includes a dielectric substrate formed of a plurality of dielectric layers; the transmission lines, the capacitors for parallel resonance, the input capacitor, the output capacitor, and the inductor for forming a pole are formed inside the dielectric substrate; and the input terminal and the output terminal are formed on a surface of the dielectric substrate.
According to this structure, since the high-frequency filter is formed of the dielectric substrate, the transmission lines, the capacitors for parallel resonance, the input capacitor, the output capacitor, and the inductor for forming a pole of the high-frequency filter can be built in the dielectric substrate. Therefore, the high-frequency filter can be made compact.
The above high-frequency filter may be configured such that the inductor for forming a pole is formed of a via-hole electrode connecting a connection electrode, which connects the other end of the input transmission line to the other end of the output transmission line formed inside the dielectric substrate, to the ground electrode formed inside the dielectric substrate.
According to this structure, since the inductor for forming a pole is formed of the via-hole electrode connecting the connection electrode, which connects the end of the input transmission line to the end of the output transmission line formed inside the dielectric substrate, to the ground electrode formed inside the dielectric substrate, the inductance of the inductor for forming a pole can be easily specified in the design stage by adjusting the length or the diameter of the via-hole electrode. Therefore, the attenuation at the pole at the higher-frequency or lower-frequency side of the center frequency and the position where the pole is formed can be easily changed in the design stage.
The above high-frequency filter may be configured such that the dielectric substrate includes first to fifth dielectric layers; the transmission lines are formed of the ground electrode formed on the upper surface of the first dielectric layer and coil electrodes formed on the upper surface of the second dielectric layer; the inductor for forming a pole is formed of a via-hole electrode passing through the second dielectric layer; the input capacitor and the output capacitor are formed of first capacitor electrodes provided for the upper surface of the third dielectric layer and shield electrodes provided for the upper surface of the fifth dielectric layer; and the capacitors for parallel resonance are formed of second capacitor electrodes provided for the upper surface of the fourth dielectric layer and the shield electrodes provided for the upper surface of the fifth dielectric layer.
According to the high-frequency filter, since the transmission lines, the capacitors for parallel resonance, and the input and output and the output capacitors of the high-frequency filter are formed inside the dielectric substrate, the transmission lines, the capacitors for parallel resonance, the input capacitor, and the output capacitor can be electrically connected with via-hole electrodes formed inside the dielectric substrate. Therefore, since it is not necessary to provide external connecting means, when the filter is mounted on a circuit board, the filter is prevented from being short-circuited to patterns on the board and other mounted components.
The present invention provides a complex electronic component in which an amplifier is mounted on the dielectric substrate constituting the above high-frequency filter.
According to this complex electronic component, since the high-frequency filter is integrated with the amplifier, the number of parts constituting a receiving circuit or a transmitting circuit can be reduced. Therefore, the receiving circuit or the transmitting circuit is made compact and inexpensive.
In addition, since the impedance of the high-frequency filter and that of the amplifier can be designed in advance so as to match each other, it is not necessary to place a device for impedance matching between the high-frequency filter and the amplifier. Therefore, the gain of the receiving circuit or the transmitting circuit can be prevented from decreasing.
The present invention also provides a portable radio apparatus including an antenna; at least one of a receiving circuit and a transmitting circuit connected to the antenna, each circuit including the above high-frequency filter; and a casing for covering at least one of the receiving circuit and the transmitting circuit.
According to this portable radio apparatus, since the high-frequency filter having a good attenuation characteristic is used for the receiving circuit or the transmitting circuit of the portable radio apparatus, the portable radio apparatus having a good transmitting or receiving characteristic is obtained.
The present invention also provides a portable radio apparatus including: an antenna; at least one of a receiving circuit and a transmitting circuit connected to the antenna, each circuit including the above complex electronic component; and a casing for covering at least one of the receiving circuit and the transmitting circuit.
According to this portable radio apparatus, since the compact complex electronic component is used for the receiving circuit or the transmitting circuit of the portable radio apparatus, the portable radio apparatus is made compact.
The present invention also provides a frequency adjustment method for the above described high-frequency filter wherein the resonant frequencies of all of a plurality of LC resonators formed of the plurality of transmission lines and the plurality of capacitors for parallel resonance are adjusted in an identical level.
According to the above described frequency adjustment method, since the resonant frequencies of all of the plurality of LC resonators formed of the plurality of transmission lines and the plurality of capacitors for parallel resonance are adjusted in the identical level, a ripple is prevented in the frequency characteristic.
In the above described frequency adjustment method, the resonant frequencies of all of the plurality of LC resonators formed of the plurality of transmission lines and the plurality of capacitors for parallel resonance may be adjusted in an identical level by trimming at least one type of electrodes of the coil electrodes constituting the plurality of transmission lines, the second capacitor electrodes constituting the plurality of capacitors for parallel resonance, and the shield electrodes constituting the plurality of capacitors for parallel resonance.
According to the above described frequency adjustment method, since the resonant frequencies of all of the plurality of LC resonators formed of the plurality of transmission lines and the plurality of capacitors for parallel resonance are adjusted in the identical level by trimming one type of electrodes of the coil electrodes constituting the plurality of transmission lines, the second capacitor electrodes constituting the plurality of capacitors for parallel resonance, and the shield electrodes constituting the plurality of capacitors for parallel resonance, the frequency adjustment of the high-frequency filter is easily performed.